The overall goal of this research is to improve the performance of composite resins as posterior restorative materials. Specifically, this goal is addressed by the systematic study of the effects of polymer cross- linking upon model restorative resin systems of controlled composition. By controlling cross-link density, it may be possible to manipulate physical properties and to design polymers for specific applications. Four specific aims are directed toward testing these hypotheses: 1. to develop a linear polymer model for the determination of cross-link determination. This preliminary step will use mono-functional molecules that are very similar to the difunctional molecules of BIS-GMA and TEGDMA to make homopolymers and blends (80/20, 70/30, and 50/50 wt/wt). The glass transition (Tg) will be determined by dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) for these linear systems. The relationship between Tg and the physical properties of surface hardness, elastic modulus, and fracture toughness will be studied. 2. to develop a model resin system to study the cross-linking potential of BIS-GM and TEGDMA and the resultant chemical and physical properties. Using the monofunctional monomer of BIS-GMA used in Specific Aim #1, co-monomers will be made of BIS-GMA and also of TEGDMA in ratios of 80/20, 70/30, and 50/50 wt/wt. A test will be developed to distinguish between the extent of cross-linking among the various formulations. A comparison of Tg values and monomer conversion of the linear system to the cross-linked model will be made. A measure of the extractable monomer remaining after cure will be correlated with cross-link density. Comparison of the effects on physical properties (hardness, elastic modulus, and fracture toughness) of changing cross-link density will be made. 3. to determine the extent of cross-linking occurring in model resin systems of BIS-GMA and TEGDMA. Co-monomer blends using only these two molecules will be made in ratios similar to those in the first two Specific Aims. Comparison of the results of cross-link density, monomer conversion, glass transition temperature, hardness, elastic modulus, and fracture toughness, will be made to those of the model system in Specific Aim #2. 4. to determine the factors affecting cross- linking and the impact that these factors have on physical properties. The following factors will be varied to study their influence on cross-link density and the resultant chemical and physical properties: main monomer type; monomer/co-monomer ratio; curing method (heat or light-cured); level of heat cure (high or low); treatment with light-curing (light-cured only or light-cured plus cost-cure heated with or without a heat-induced free radical generator); and the effect of concentration on free radical generator. Chemical property evaluation will consist of cross-link density, Tg, and monomer conversion. Physical property evaluation will consist of hardness, and fracture toughness. Correlation of the different test variables will be made to the different chemical and physical tests to determine the influence of formulation and cure mode on these parameters.